To edit a translation, either select it and click Edit or just double-click the item in the list. If you have more than one item selected when you click Edit, the VS will open only the first selected item in the table for editing; you cannot edit multiple items at the same time.
The Edit Translation dialog, shown below, displays the current settings for the selected item and allows you to change either the modifier or the translation type or both.
As shown, the Translate From setting cannot be changed since you are editing an existing translation, not adding a new one.
When finished, click OK to save your changes.